RFC 6424

Internet Engineering Task Force (IETF) N. Bahadur
Request for Comments: 6424 K. Kompella
Updates: 4379 Juniper Networks, Inc.
Category: Standards Track G. Swallow
ISSN: 2070-1721 Cisco Systems
November 2011 Mechanism for Performing Label Switched Path Ping (LSP Ping)
over MPLS Tunnels
Abstract
This document describes methods for performing LSP ping (specified in
RFC 4379) traceroute over MPLS tunnels and for traceroute of stitched
MPLS Label Switched Paths (LSPs). The techniques outlined in RFC
4379 are insufficient to perform traceroute Forwarding Equivalency
Class (FEC) validation and path discovery for an LSP that goes over
other MPLS tunnels or for a stitched LSP. This document deprecates
the Downstream Mapping TLV (defined in RFC 4379) in favor of a new
TLV that, along with other procedures outlined in this document, can
be used to trace such LSPs.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6424.

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1. Introduction
This documents describes methods for performing LSP ping (specified
in [RFC4379]) traceroute over MPLS tunnels. The techniques in
[RFC4379] outline a traceroute mechanism that includes Forwarding
Equivalency Class (FEC) validation and Equal Cost Multi-Path (ECMP)
path discovery. Those mechanisms are insufficient and do not provide
details when the FEC being traced traverses one or more MPLS tunnels
and when Label Switched Path (LSP) stitching [RFC5150] is in use.
This document deprecates the Downstream Mapping TLV [RFC4379],
introducing instead a new TLV that is more extensible and that
enables retrieval of detailed information. Using the new TLV format
along with the existing definitions of [RFC4379], this document
describes procedures by which a traceroute request can correctly
traverse MPLS tunnels with proper FEC and label validations.
1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Motivation
An LSP ping traceroute may cross multiple MPLS tunnels en route to
the destination. Let us consider a simple case.
A B C D E
o -------- o -------- o --------- o --------- o
\_____/ | \______/ \______/ | \______/
LDP | RSVP RSVP | LDP
| |
\____________________/
LDP
Figure 1: LDP over RSVP Tunnel
When a traceroute is initiated from router A, router B returns
downstream mapping information for node C in the MPLS echo reply.
The next MPLS echo request reaches router C with an LDP FEC. Node C
is a pure RSVP node and does not run LDP. Node C will receive the
MPLS echo request with two labels but only one FEC in the Target FEC
stack. Consequently, node C will be unable to perform a complete FEC
validation. It will let the trace continue by just providing next-
hop information based on the incoming label, and by looking up the
forwarding state associated with that label. However, ignoring FEC
validation defeats the purpose of control-plane validations. The

MPLS echo request should contain sufficient information to allow node
C to perform FEC validations to catch any misrouted echo requests.
The above problem can be extended for a generic case of hierarchical
tunnels or stitched tunnels (e.g., B-C can be a separate RSVP tunnel
and C-D can be a separate RSVP tunnel). The problem of FEC
validation for tunnels can be solved if the transit routers (router B
in the above example) provide some information to the ingress
regarding the start of a new tunnel.
Stitched LSPs involve two or more LSP segments stitched together.
The LSP segments can be signaled using the same or different
signaling protocols. In order to perform an end-to-end trace of a
stitched LSP, the ingress needs to know FEC information regarding
each of the stitched LSP segments. For example, consider the figure
below.
A B C D E F
o -------- o -------- o --------- o -------- o ------- o
\_____/ \______/ \______/ \______/ \_______/
LDP LDP BGP RSVP RSVP
Figure 2: Stitched LSP
Consider ingress (A) tracing end-to-end stitched LSP A--F. When an
MPLS echo request reaches router C, there is a FEC stack change
happening at router C. With current LSP ping [RFC4379] mechanisms,
there is no way to convey this information to A. Consequently, when
the next echo request reaches router D, router D will know nothing
about the LDP FEC that A is trying to trace.
Thus, the procedures defined in [RFC4379] do not make it possible for
the ingress node to:
1. Know that tunneling has occurred.
2. Trace the path of the tunnel.
3. Trace the path of stitched LSPs.
3. Packet Format
3.1. Summary of Changes
In many cases, there is a need to associate additional data in the
MPLS echo reply. In most cases, the additional data needs to be
associated on a per-downstream-neighbor basis. Currently, the MPLS
echo reply contains one Downstream Mapping TLV (DSMAP) per downstream

neighbor. However, the DSMAP format is not extensible; hence, it is
not possible to associate more information with a downstream
neighbor. This document defines a new extensible format for the
DSMAP and provides mechanisms for solving the tunneled LSP ping
problem using the new format. In summary, this document makes the
following TLV changes:
o Addition of new Downstream Detailed Mapping TLV (DDMAP).
o Deprecation of existing Downstream Mapping TLV (DSMAP).
o Addition of Downstream FEC stack change sub-TLV to DDMAP.
3.2. New Return Codes
3.2.1. Return Code per Downstream
A new Return Code is being defined "See DDM TLV for Return Code and
Return Subcode" (Section 6.3) to indicate that the Return Code is per
Downstream Detailed Mapping TLV (Section 3.3). This Return Code MUST
be used only in the message header and MUST be set only in the MPLS
echo reply message. If the Return Code is set in the MPLS echo
request message, then it MUST be ignored. When this Return Code is
set, each Downstream Detailed Mapping TLV MUST have an appropriate
Return Code and Return Subcode. This Return Code MUST be used when
there are multiple downstreams for a given node (such as Point to
Multipoint (P2MP) or Equal Cost Multi-Path (ECMP)), and the node
needs to return a Return Code/Return Subcode for each downstream.
This Return Code MAY be used even when there is only one downstream
for a given node.
3.2.2. Return Code for Stitched LSPs
When a traceroute is being performed on stitched LSPs
(Section 4.1.2), the stitching point SHOULD indicate the stitching
action to the node performing the trace. This is done by setting the
Return Code to "Label switched with FEC change" (Section 6.3). If a
node is performing FEC hiding, then it MAY choose to set the Return
Code to a value (specified in [RFC4379]) other than "Label switched
with FEC change". The Return Code "Label switched with FEC change"
MUST NOT be used if no FEC stack sub-TLV (Section 3.3.1.3) is present
in the Downstream Detailed Mapping TLV(s). This new Return Code MAY
be used for hierarchical LSPs (for indicating the start or end of an
outer LSP).

Address Type
The Address Type indicates if the interface is numbered or
unnumbered. It also determines the length of the Downstream IP
Address and Downstream Interface fields.
DS Flags
The DS Flags field is a bit vector of various flags.
Downstream Address and Downstream Interface Address
IPv4 addresses and interface indices are encoded in 4 octets; IPv6
addresses are encoded in 16 octets. For details regarding setting
the address value, refer to [RFC4379].
The newly added sub-TLVs and their fields are as described below.
Return Code
The Return Code is set to zero by the sender. The receiver can
set it to one of the values specified in the "Multi-Protocol Label
Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters"
registry, "Return Codes" sub-registry.
If the receiver sets a non-zero value of the Return Code field in
the Downstream Detailed Mapping TLV, then the receiver MUST also
set the Return Code field in the echo reply header to "See DDM TLV
for Return Code and Return Subcode" (Section 6.3). An exception
to this is if the receiver is a bud node [RFC4461] and is replying
as both an egress and a transit node with a Return Code of 3
("Replying router is an egress for the FEC at stack-depth <RSC>")
in the echo reply header.
If the Return Code of the echo reply message is not set to either
"See DDM TLV for Return Code and Return Subcode" (Section 6.3) or
"Replying router is an egress for the FEC at stack-depth <RSC>",
then the Return Code specified in the Downstream Detailed Mapping
TLV MUST be ignored.
Return Subcode
The Return Subcode is set to zero by the sender. The receiver can
set it to one of the values specified in the "Multi-Protocol Label
Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters"
registry, "Return Codes" sub-registry. This field is filled in
with the stack-depth for those codes that specify the stack-depth.
For all other codes, the Return Subcode MUST be set to zero.

MBZ
MUST be set to zero when sending; MUST be ignored on receipt.
Multipath Information
Encoded multipath data, according to the Multipath Type.
3.3.1.2. Label Stack Sub-TLV
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Downstream Label | Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Downstream Label | Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Label Stack Sub-TLV
The Label stack sub-TLV contains the set of labels in the label stack
as it would have appeared if this router were forwarding the packet
through this interface. Any Implicit Null labels are explicitly
included. The number of label/protocol pairs present in the sub-TLV
is determined based on the sub-TLV data length. The label format and
protocol type are as defined in [RFC4379]. When the Downstream
Detailed Mapping TLV is sent in the echo reply, this sub-TLV MUST be
included.
Downstream Label
A Downstream label is 24 bits, in the same format as an MPLS label
minus the Time to Live (TTL) field, i.e., the MSBit of the label
is bit 0, the LSBit is bit 19, the Traffic Class (TC) field
[RFC5462] is bits 20-22, and S is bit 23. The replying router
SHOULD fill in the TC field and S bit; the LSR receiving the echo
reply MAY choose to ignore these.
Protocol
This specifies the label distribution protocol for the Downstream
label.

3.3.1.3. FEC Stack Change Sub-TLV
A router MUST include the FEC stack change sub-TLV when the
downstream node in the echo reply has a different FEC Stack than the
FEC Stack received in the echo request. One or more FEC stack change
sub-TLVs MAY be present in the Downstream Detailed Mapping TLV. The
format is as below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Operation Type | Address Type | FEC-tlv length| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote Peer Address (0, 4 or 16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. FEC TLV .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: FEC Stack Change Sub-TLV
Operation Type
The operation type specifies the action associated with the FEC
stack change. The following operation types are defined:
Type # Operation
------ ---------
1 Push
2 Pop
Address Type
The Address Type indicates the remote peer's address type. The
Address Type is set to one of the following values. The length of
the peer address is determined based on the address type. The
address type MAY be different from the address type included in
the Downstream Detailed Mapping TLV. This can happen when the LSP
goes over a tunnel of a different address family. The address
type MAY be set to Unspecified if the peer address is either
unavailable or the transit router does not wish to provide it for
security or administrative reasons.

Type # Address Type Address length
------ ------------ --------------
0 Unspecified 0
1 IPv4 4
2 IPv6 16
FEC TLV Length
Length in bytes of the FEC TLV.
Reserved
This field is reserved for future use and MUST be set to zero.
Remote Peer Address
The remote peer address specifies the remote peer that is the
next-hop for the FEC being currently traced. For example, in the
LDP over RSVP case in Figure 1, router B would respond back with
the address of router D as the remote peer address for the LDP FEC
being traced. This allows the ingress node to provide information
regarding FEC peers. If the operation type is PUSH, the remote
peer address is the address of the peer from which the FEC being
pushed was learned. If the operation type is POP, the remote peer
address MAY be set to Unspecified.
For upstream-assigned labels [RFC5331], an operation type of POP
will have a remote peer address (the upstream node that assigned
the label) and this SHOULD be included in the FEC stack change
sub-TLV. The remote peer address MAY be set to Unspecified if the
address needs to be hidden.
FEC TLV
The FEC TLV is present only when the FEC-tlv length field is non-
zero. The FEC TLV specifies the FEC associated with the FEC stack
change operation. This TLV MAY be included when the operation
type is POP. It MUST be included when the operation type is PUSH.
The FEC TLV contains exactly one FEC from the list of FECs
specified in [RFC4379]. A Nil FEC MAY be associated with a PUSH
operation if the responding router wishes to hide the details of
the FEC being pushed.

FEC stack change sub-TLV operation rules are as follows:
a. A FEC stack change sub-TLV containing a PUSH operation MUST NOT
be followed by a FEC stack change sub-TLV containing a POP
operation.
b. One or more POP operations MAY be followed by one or more PUSH
operations.
c. One FEC stack change sub-TLV MUST be included per FEC stack
change. For example, if 2 labels are going to be pushed, then
one FEC stack change sub-TLV MUST be included for each FEC.
d. A FEC splice operation (an operation where one FEC ends and
another FEC starts, see Figure 7) MUST be performed by including
a POP type FEC stack change sub-TLV followed by a PUSH type FEC
stack change sub-TLV.
e. A Downstream detailed mapping TLV containing only one FEC stack
change sub-TLV with Pop operation is equivalent to IS_EGRESS
(Return Code 3, [RFC4379]) for the outermost FEC in the FEC
stack. The ingress router performing the MPLS traceroute MUST
treat such a case as an IS_EGRESS for the outermost FEC.
3.4. Deprecation of Downstream Mapping TLV
This document deprecates the Downstream Mapping TLV. LSP ping
procedures should now use the Downstream Detailed Mapping TLV.
Detailed procedures regarding interoperability between the deprecated
TLV and the new TLV are specified in Section 4.4.
4. Performing MPLS Traceroute on Tunnels
This section describes the procedures to be followed by an LSP
ingress node and LSP transit nodes when performing MPLS traceroute
over MPLS tunnels.
4.1. Transit Node Procedure
4.1.1. Addition of a New Tunnel
A transit node (Figure 1) knows when the FEC being traced is going to
enter a tunnel at that node. Thus, it knows about the new outer FEC.
All transit nodes that are the origination point of a new tunnel
SHOULD add the FEC stack change sub-TLV (Section 3.3.1.3) to the
Downstream Detailed Mapping TLV (Figure 3) in the echo reply. The
transit node SHOULD add one FEC stack change sub-TLV of operation
type PUSH, per new tunnel being originated at the transit node.

A transit node that sends a Downstream FEC stack change sub-TLV in
the echo reply SHOULD fill the address of the remote peer; which is
the peer of the current LSP being traced. If the transit node does
not know the address of the remote peer, it MUST set the address type
to Unspecified.
The Label stack sub-TLV MUST contain one additional label per FEC
being PUSHed. The label MUST be encoded as per Figure 5. The label
value MUST be the value used to switch the data traffic. If the
tunnel is a transparent pipe to the node, i.e. the data-plane trace
will not expire in the middle of the new tunnel, then a FEC stack
change sub-TLV SHOULD NOT be added and the Label stack sub-TLV SHOULD
NOT contain a label corresponding to the hidden tunnel.
If the transit node wishes to hide the nature of the tunnel from the
ingress of the echo request, then it MAY not want to send details
about the new tunnel FEC to the ingress. In such a case, the transit
node SHOULD use the Nil FEC. The echo reply would then contain a FEC
stack change sub-TLV with operation type PUSH and a Nil FEC. The
value of the label in the Nil FEC MUST be set to zero. The remote
peer address type MUST be set to Unspecified. The transit node
SHOULD add one FEC stack change sub-TLV of operation type PUSH, per
new tunnel being originated at the transit node. The Label stack
sub-TLV MUST contain one additional label per FEC being PUSHed. The
label value MUST be the value used to switch the data traffic.
4.1.2. Transition between Tunnels
A B C D E F
o -------- o -------- o --------- o -------- o ------- o
\_____/ \______/ \______/ \______/ \_______/
LDP LDP BGP RSVP RSVP
Figure 7: Stitched LSPs
In the above figure, we have three separate LSP segments stitched at
C and D. Node C SHOULD include two FEC stack change sub-TLVs. One
with a POP operation for the LDP FEC and one with the PUSH operation
for the BGP FEC. Similarly, node D SHOULD include two FEC stack
change sub-TLVs, one with a POP operation for the BGP FEC and one
with a PUSH operation for the RSVP FEC. Nodes C and D SHOULD set the
Return Code to "Label switched with FEC change" (Section 6.3) to
indicate change in FEC being traced.
If node C wishes to perform FEC hiding, it SHOULD respond back with
two FEC stack change sub-TLVs, one POP followed by one PUSH. The POP
operation MAY either exclude the FEC TLV (by setting the FEC TLV
length to 0) or set the FEC TLV to contain the LDP FEC. The PUSH

operation SHOULD have the FEC TLV containing the Nil FEC. The Return
Code SHOULD be set to "Label switched with FEC change".
If node C performs FEC hiding and node D also performs FEC hiding,
then node D MAY choose to not send any FEC stack change sub-TLVs in
the echo reply since the number of labels has not changed (for the
downstream of node D) and the FEC type also has not changed (Nil
FEC). In such a case, node D MUST NOT set the Return Code to "Label
switched with FEC change". If node D performs FEC hiding, then node
F will respond as IS_EGRESS for the Nil FEC. The ingress (node A)
will know that IS_EGRESS corresponds to the end-to-end LSP.
A B C D E F
o -------- o -------- o --------- o --------- o --------- o
\_____/ |\____________________/ |\_______/
LDP |\ RSVP-A | LDP
| \_______________________________/|
| RSVP-B |
\________________________________/
LDP
Figure 8: Hierarchical LSPs
In the above figure, we have an end-to-end LDP LSP between nodes A
and F. The LDP LSP goes over RSVP LSP RSVP-B. LSP RSVP-B itself
goes over another RSVP LSP RSVP-A. When node A initiates a
traceroute for the end-to-end LDP LSP, then following sequence of FEC
stack change sub-TLVs will be performed
Node B:
Respond with two FEC stack change sub-TLVs: PUSH RSVP-B, PUSH RSVP-A.
Node D:
Respond with Return Code 3 when RSVP-A is the top of FEC stack. When
the echo request contains RSVP-B as top of stack, respond with
Downstream information for node E and an appropriate Return Code.

If node B is performing tunnel hiding, then:
Node B:
Respond with two FEC stack change sub-TLVs: PUSH Nil FEC, PUSH Nil
FEC.
Node D:
If D determines that the Nil FEC corresponds to RSVP-A, which
terminates at D, then it SHOULD respond with Return Code 3. D can
also respond with FEC stack change sub-TLV: POP (since D knows that
number of labels towards next-hop is decreasing). Both responses
would be valid.
A B C D E F G
o -------- o -------- o ------ o ------ o ----- o ----- o
LDP LDP BGP \ RSVP RSVP / LDP
\_____________/
LDP
Figure 9: Stitched Hierarchical LSPs
In the above case, node D will send three FEC stack change sub-TLVs.
One POP (for the BGP FEC) followed by two PUSHes (one for LDP and one
for RSVP). Nodes C and D SHOULD set the Return Code to "Label
switched with FEC change" (Section 6.3) to indicate change in FEC
being traced.
4.1.3. Modification to FEC Validation Procedure on Transit
Section 4.4 of [RFC4379] specifies Target FEC stack validation
procedures. This document enhances the FEC validation procedures as
follows. If the outermost FEC of the target FEC stack is the Nil
FEC, then the node MUST skip the target FEC validation completely.
This is to support FEC hiding, in which the outer hidden FEC can be
the Nil FEC.
4.2. Modification to FEC Validation Procedure on Egress
Section 4.4 of [RFC4379] specifies Target FEC stack validation
procedures. This document enhances the FEC validation procedures as
follows. If the outermost FEC of the Target FEC stack is the Nil
FEC, then the node MUST skip the target FEC validation completely.
This is to support FEC hiding, in which the outer hidden FEC can be
the Nil FEC.

4.3. Ingress Node Procedure
It is the responsibility of an ingress node to understand tunnel
within tunnel semantics and LSP stitching semantics when performing a
MPLS traceroute. This section describes the ingress node procedure
based on the kind of reply an ingress node receives from a transit
node.
4.3.1. Processing Downstream Detailed Mapping TLV
Downstream Detailed Mapping TLV should be processed in the same way
as the Downstream Mapping TLV, defined in Section 4.4 of [RFC4379].
This section describes the procedures for processing the new elements
introduced in this document.
4.3.1.1. Stack Change Sub-TLV Not Present
This would be the default behavior as described in [RFC4379]. The
ingress node MUST perform MPLS echo reply processing as per the
procedures in [RFC4379].
4.3.1.2. Stack Change Sub-TLV(s) Present
If one or more FEC stack change sub-TLVs (Section 3.3.1.3) are
received in the MPLS echo reply, the ingress node SHOULD process them
and perform some validation.
The FEC stack changes are associated with a downstream neighbor and
along a particular path of the LSP. Consequently, the ingress will
need to maintain a FEC stack per path being traced (in case of
multipath). All changes to the FEC stack resulting from the
processing of FEC stack change sub-TLV(s) should be applied only for
the path along a given downstream neighbor. The following algorithm
should be followed for processing FEC stack change sub-TLVs.

The next MPLS echo request along the same path should use the
modified FEC stack obtained after processing the FEC stack change
sub-TLVs. A non-Nil FEC guarantees that the next echo request along
the same path will have the Downstream Detailed Mapping TLV validated
for IP address, Interface address, and label stack mismatches.
If the top of the FEC stack is a Nil FEC and the MPLS echo reply does
not contain any FEC stack change sub-TLVs, then it does not
necessarily mean that the LSP has not started traversing a different
tunnel. It could be that the LSP associated with the Nil FEC
terminated at a transit node and at the same time a new LSP started
at the same transit node. The Nil FEC would now be associated with
the new LSP (and the ingress has no way of knowing this). Thus, it
is not possible to build an accurate hierarchical LSP topology if a
traceroute contains Nil FECs.
4.3.2. Modifications to Handling a Return Code 3 Reply.
The procedures above allow the addition of new FECs to the original
FEC being traced. Consequently, a reply from a downstream node with
Return Code 3 (IS_EGRESS) may not necessarily be for the FEC being
traced. It could be for one of the new FECs that was added. On
receipt of an IS_EGRESS reply, the LSP ingress should check if the
depth of Target FEC sent to the node that just responded, was the
same as the depth of the FEC that was being traced. If it was not,
then it should pop an entry from the Target FEC stack and resend the
request with the same TTL (as previously sent). The process of
popping a FEC is to be repeated until either the LSP ingress receives
a non-IS_EGRESS reply or until all the additional FECs added to the
FEC stack have already been popped. Using an IS_EGRESS reply, an
ingress can build a map of the hierarchical LSP structure traversed
by a given FEC.
4.3.3. Handling of New Return Codes
When the MPLS echo reply Return Code is "Label switched with FEC
change" (Section 3.2.2), the ingress node SHOULD manipulate the FEC
stack as per the FEC stack change sub-TLVs contained in the
downstream detailed mapping TLV. A transit node can use this Return
Code for stitched LSPs and for hierarchical LSPs. In case of ECMP or
P2MP, there could be multiple paths and Downstream Detailed Mapping
TLVs with different Return Codes (Section 3.2.1). The ingress node
should build the topology based on the Return Code per ECMP path/P2MP
branch.

4.4. Handling Deprecated Downstream Mapping TLV
The Downstream Mapping TLV has been deprecated. Applications should
now use the Downstream Detailed Mapping TLV. The following
procedures SHOULD be used for backward compatibility with routers
that do not support the Downstream Detailed Mapping TLV.
o The Downstream Mapping TLV and the Downstream Detailed Mapping TLV
MUST never be sent together in the same MPLS echo request or in
the same MPLS echo reply.
o If the echo request contains a Downstream Detailed Mapping TLV and
the corresponding echo reply contains a Return Code 2 ("One or
more of the TLVs was not understood"), then the sender of the echo
request MAY resend the echo request with the Downstream Mapping
TLV (instead of the Downstream Detailed Mapping TLV). In cases
where a detailed reply is needed, the sender can choose to ignore
the router that does not support the Downstream Detailed Mapping
TLV.
o If the echo request contains a Downstream Mapping TLV, then a
Downstream Detailed Mapping TLV MUST NOT be sent in the echo
reply. This is to handle the case that the sender of the echo
request does not support the new TLV. The echo reply MAY contain
Downstream Mapping TLV(s).
o If echo request forwarding is in use (such that the echo request
is processed at an intermediate LSR and then forwarded on), then
the intermediate router is responsible for making sure that the
TLVs being used among the ingress, intermediate and destination
are consistent. The intermediate router MUST NOT forward an echo
request or an echo reply containing a Downstream Detailed Mapping
TLV if it itself does not support that TLV.
5. Security Considerations
1. If a network operator wants to prevent tracing inside a tunnel,
one can use the Pipe Model [RFC3443], i.e., hide the outer MPLS
tunnel by not propagating the MPLS TTL into the outer tunnel (at
the start of the outer tunnel). By doing this, MPLS traceroute
packets will not expire in the outer tunnel and the outer tunnel
will not get traced.
2. If one doesn't wish to expose the details of the new outer LSP,
then the Nil FEC can be used to hide those details. Using the
Nil FEC ensures that the trace progresses without false negatives
and all transit nodes (of the new outer tunnel) perform some
minimal validations on the received MPLS echo requests.

Other security considerations, as discussed in [RFC4379], are also
applicable to this document.
6. IANA Considerations
6.1. New TLV
IANA has assigned a TLV type value to the following TLV from the
"Multiprotocol Label Switching Architecture (MPLS) Label Switched
Paths (LSPs) Ping Parameters" registry, "TLVs and sub-TLVs" sub-
registry.
Downstream Detailed Mapping TLV (see Section 3.3): 20.
6.2. New Sub-TLV Types and Associated Registry
IANA has registered the Sub-Type field of Downstream Detailed Mapping
TLV. The valid range for this is 0-65535. Assignments in the range
0-16383 and 32768-49161 are made via Standards Action as defined in
[RFC3692]; assignments in the range 16384-31743 and 49162-64511 are
made via Specification Required [RFC4379]; values in the range 31744-
32767 and 64512-65535 are for Vendor Private Use, and MUST NOT be
allocated. If a sub-TLV has a Type that falls in the range for
Vendor Private Use, the Length MUST be at least 4, and the first four
octets MUST be that vendor's SMI Enterprise Code, in network octet
order. The rest of the Value field is private to the vendor.
IANA has assigned the following sub-TLV types (see Section 3.3.1):
Multipath data: 1
Label stack: 2
FEC stack change: 3
6.3. New Return Codes
IANA has assigned new Return Code values from the "Multi-Protocol
Label Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters"
registry, "Return Codes" sub-registry, as follows using a Standards
Action value.
Value Meaning
----- -------
14 See DDM TLV for Return Code and Return Subcode
15 Label switched with FEC change